This invention relates to a solid electrolyte for high-temperature electrolysis having a multi-layer electrode applied thereto.
High-temperature electrolyzers are electrochemical converters which generate hydrogen from water vapor while using electrical energy with a high efficiency. A basic diagram of such an electrolyzer is shown in FIG. 1. Because their efficiency is greater than 90%, such devices are very important for a future hydrogen energy management cycle which is based on non-fossil primary energy sources, such as solar and nuclear energy. It is contemplated that the hydrogen from such systems may be used to replace previously used storable fossil fuel energy carriers, such as natural gas, gasoline, diesel oil, etc.
With respect to economic efficiency and high energy density, a planar cell design is advantageous. In such an arrangement, thin solid electrolytic plates are coated on both sides with porous electrodes, and the coated plates are stacked alternately with connecting elements above one another so that a bipolar arrangement is created. Several individual cells may be connected in series. The operating temperature is approximately 800.degree. to 1,000.degree. C.
The aforementioned high-temperature electrolyzer arrangements may also be used for the reverse process of fuel cell operation in order to produce electric energy with a high degree of efficiency from fuel gases such as hydrogen, carbon monoxide or methane.
The following materials are known for use as components of the high-temperature electrolyzers:
Oxygen Electrode (Air Electrode): PA1 Electrolyte: PA1 Hydrogen Electrode (Fuel Gas Electrode): PA1 Connecting Element: PA1 an upper electrode layer consisting of an electronically conductive oxidic material, and PA1 an intermediate layer of an Al.sub.2 O.sub.3 -free, at least partially stabilized zirconium oxide arranged between said upper electrode layer and said solid electrolyte. PA1 an upper gas-permeable electrode layer made of an electrically conductive ceramic/metal particle composite material comprising at least one metal selected from the group consisting of nickel and cobalt, and at least one ceramic material selected from the group consisting of doped zirconium oxide and doped ceroxide, and PA1 an Al.sub.2 O.sub.3 -free intermediate layer of an at least partially stabilized zirconium oxide interposed between said upper gas-permeable electrode layer and said solid electrolyte. PA1 1. Oxygen Electrode: PA1 2. Mixed Conductive Intermediate Layer: PA1 3. Al.sub.2 O.sub.3 -Free ZrO.sub.2 Intermediate Layer: PA1 4. Solid Electrolyte:
Doped oxides with a perovskite structure such as La.sub.1-x Ca.sub.x MnO.sub.3, La.sub.1-x Sr.sub.x MnO.sub.3 or La.sub.1-x Sr.sub.x Co.sub.y Mn.sub.1-y O.sub.3. PA2 Partially or fully stabilized ZrO.sub.2 doped with CaO--, MgO--, Y.sub.2 O.sub.3 -- or other rare-earth oxide. PA2 Ceramic/metal composite materials with nickel or cobalt as the metallic component and doped CeO.sub.2 or ZrO.sub.2 as the ceramic component. PA2 Doped lanthanum chromite such as La.sub.1-x Sr.sub.x CrO.sub.3 or LaMg.sub.x Cr.sub.1-x O.sub.3. PA2 porous mixed oxide with a perovskite structure, such as La.sub.1-x Ca.sub.x MnO.sub.3, wherein 0.05.ltoreq.x.ltoreq.0.7, or La.sub.1-x Sr.sub.x MnO.sub.3, wherein 0.05.ltoreq.x.ltoreq.0.7. PA2 mixture of ion-conducting material and electron-conducting material, such as (ZrO.sub.2).sub.1-y (Y.sub.2 O.sub.3).sub.y, wherein 0.0.ltoreq.y.ltoreq.0.2, and La.sub.1-x Ca.sub.x MnO.sub.3, wherein 0.05.ltoreq.x.ltoreq.0.5. PA2 partially or fully stabilized ZrO.sub.2, for example, fully stabilized with 8 to 12 mole-% Y.sub.2 O.sub.3. PA2 partially or fully stabilized ZrO.sub.2 with added Al.sub.2 O.sub.3, for example [(ZrO.sub.2).sub.1-a (Y.sub.2 O.sub.3).sub.a ].sub.1-b [Al.sub.2 O.sub.3 ].sub.b, wherein 0.03.ltoreq.a.ltoreq.0.20, and 0.01.ltoreq.b.ltoreq.0.25.
Solid electrolytes of fully or partially stabilized ZrO.sub.2 with additions of Al.sub.2 O.sub.3 are known from German Patent No. DE 2,852,647 C2 and Examined German Patent Application No. DE 1,671,704 B2. Oxygen electrodes made of an electronically conductive oxidic material are known from Published German Patent Application Nos. DE 4,136,448 A1 and DE 3,935,310 A1.
Fuel gas electrodes made of a ceramic/metal particle composite material are also known from Published German Patent Application Nos. DE 39 35 310 A1 and DE 39 22 673 A1. Examined German Patent Application No. DE 17 71 829 B2, discloses a multi-layer electrode on a solid electrolyte in the case of which the layer which is adjacent to the solid electrolyte consists of individual grains made of partially or fully stabilized ZrO.sub.2.
A solid electrolyte made of partially or fully stabilized ZrO.sub.2 with Y.sub.2 O.sub.3 doping and an addition of Al.sub.2 O.sub.3 is described in Published German Patent Application No. DE 2,904,069 A1. The associated oxygen electrode consists of an oxidic material. This combination of the electrode and the solid electrolyte represents a starting point for the present invention. Published German Patent Application No. DE 29 04 069 A1, discloses an arrangement in which an intermediate layer of fully stabilized ZrO.sub.2 with Al.sub.2 O.sub.3 is disposed between the electrode and the solid electrolyte.
Despite all the efforts of the prior art, there remains a need for improved solid electrolytes with attached multi-layer electrodes.